Skin sympathetic nerve activity: Potential new non-invasive indicator of exercise metabolic thresholds in adults

This study aims to explore the role of Skin Sympathetic Nerve Activity (SKNA) in exercise monitoring and the determination of metabolic thresholds, and to verify the reliability of the SKNA Thresholds (SKNATs). During the Ramp Incremental Exercise Test (RIET), aSKNA, HR, and gas metabolic parameters were recorded and analyzed to assess their changes and the feasibility of SKNATs. Results showed that, compared to the baseline, aSKNA significantly increased during RIET (p < 0.001), and decreased after exercise. The first SKNA threshold (SKNAT₁) occurred at 43 ± 6% of exercise time, and the second SKNA threshold (SKNAT₂) occurred at 76 ± 5%. At SKNAT₁, both HR and aSKNA were significantly lower than at the first ventilatory threshold (VT₁) (p < 0.05, p < 0.01), while at SKNAT₂, all measures except aSKNA were significantly lower than at the second ventilatory threshold (VT₂) (p < 0.05). A strong correlation was observed between SKNATs and VTs, with the highest correlation found at VO₂ (r = 0.902, p < 0.0001). ICC showed moderate to strong concordance for all parameters at SKNAT1 and VT1, with higher agreement at the second thresholds. Collectively, aSKNA increases with exercise load during RIET and exhibits non-linear inflection points. SKNATs show strong potential for predicting VTs, representing a promising emerging indicator for determining metabolic thresholds.